1. Field of the Invention
The invention relates to a progressive-scan image sensor used for a computer inputting camera and a digital still camera, and also to a method of driving the same.
2. Description of the Related Art
In recent years, an image-inputting device such as a computer inputting camera and a digital still camera has been actively developed. Since an image sensor used for such an image inputting device is required to have images having high vertical resolution by single exposure, it is preferable to adopt a progressive-scan type in place of an interlace type for scanning a screen. A progressive-scan image sensor is designed to include at least three independent transfer electrodes per a unit pixel in order to read out signal charges in all pixels at a time. In a conventional method, at least three-phase driving pulse signals are applied to those transfer electrodes.
An example of a progressive-scan image sensor is found in "A 1/2-in 330K Progressive-Scan CCD Image Sensor with Square-Pixel", K. Nakashima et al., ITE Technical Report, Vol. 18, No. 67, pp. 7-12, November 1994, or in "A 1/3-inch 330K Square-Pixel Progressive-Scan IT-CCD Image Sensor", T. Okutani et al., 1995 ITE Annual Convention, pp. 93-94. As described in those reports, an image sensor is characterized to have a transfer electrode comprised of a three-layered gate electrode film.
Another example of a progressive-scan image sensor has been suggested in Japanese Patent Publication No. 8-21705. The suggested progressive-scan image sensor is a charge coupled type image sensor where a transfer electrode is designed to have an island shape, and an electrically conductive wiring is connected to the island-shaped transfer electrode.
Hereinbelow is explained the firstly mentioned conventional progressive-scan image sensor with reference to FIGS. 1A to 1C. FIG. 1A is a schematic view illustrating the firstly mentioned conventional progressive-scan image sensor, FIG. 1B is a cross-sectional view taken along the line IB--IB, and FIG. 1C is a cross-sectional view taken along the line IC--IC in FIG. 1A.
The illustrated image sensor is of an interline type image sensor comprising photodiode regions 610, and charge transfer regions 611 for reading signal charges photoelectrically converted in the photodiode regions 610, and transferring the thus photoelectrically converted signal charges in a vertical transfer direction X1. The photodiode regions 610 are spaced away from one another in the direction X1. Hereinafter, spaces formed between the photodiode regions 610 in the direction X1 are referred to as connection portions 612.
Each of transfer electrodes is comprised of three layers per a pixel. Namely, as illustrated in FIGS. 1B and 1C, transfer electrodes 621, 622 and 623 comprised of first, second and third layers, respectively, are formed on a semiconductor substrate 625 in the charge transfer regions 611 and the connection portions 612 with a gate insulating film 626 sandwiched between the transfer electrodes 621, 622, 623 and the semiconductor substrate 625.
Between the transfer electrodes 621, 622 and 623 are formed an interlayer insulating film 641 such as a thermal oxide film. All the transfer electrodes 621, 622 and 623 extend through the connection portions 612, and make electrical connection with a bus line (not illustrated) disposed around the semiconductor substrate 625.
When charges are transferred, a first phase driving pulse .phi.1 is applied to the transfer electrode 621 made of the first layer, a second phase driving pulse .phi.2 is applied to the transfer electrode 622 made of the second layer, and a third phase driving pulse .phi.3 is applied to the transfer electrode 623 made of the third layer. When a signal charge is read out from the photodiode region 610 to the charge transfer region 611, a read-out pulse .phi.TG in addition to a driving pulse is applied to one of the three transfer electrodes 621, 622 and 623, for instance, to the transfer electrode 622 made of the second layer.
As illustrated in FIGS. 1B and 1C, the three transfer electrodes 621, 622 and 623 and the semiconductor substrate 625 are covered with an interlayer insulating film 642 such as a thermal oxide film and an oxide film formed by chemical vapor deposition (hereinafter, referred to simply as "CVD"). In addition, a photoshield film 624 is formed on the interlayer insulating film 642 above the transfer electrodes 621, 622 and 623 for preventing lights from directly entering the charge transfer regions 611. As illustrated in FIG. 1C, the photoshield film 624 is formed with openings 628 between the transfer electrodes 621, 622 and 623 in the direction X1. The photoshield film 624 is grounded, or is designed to be fixed at a certain voltage, and has a function of only preventing lights from passing therethrough.
Thus, the firstly mentioned conventional progressive-scan image sensor has provided a three-layered electrode, three-phase driving type progressive-scan image sensor.
Hereinbelow is explained the secondly mentioned conventional progressive-scan image sensor with reference to FIGS. 2A to 2C. FIG. 2A is a schematic view illustrating the secondly mentioned conventional progressive-scan image sensor, FIG. 2B is a cross-sectional view taken along the line IIB--IIB, and FIG. 2C is a cross-sectional view taken along the line IIC--IIC in FIG. 2A.
The illustrated image sensor is of an interline type image sensor comprising photodiode regions 710, and charge transfer regions 711 for reading signal charges photoelectrically converted in the photodiode regions 710, and transferring the thus photoelectrically converted signal charges in a vertical transfer direction X1. The photodiode regions 710 are spaced away from one another in the direction X1. Hereinafter, spaces formed between the photodiode regions 710 in the direction X1 are referred to as connection portions 712.
Each of transfer electrodes is comprised of four sections per a pixel. Namely, as illustrated in FIGS. 2B and 2C, a transfer electrode 721 made of a first layer, transfer electrodes 722a and 722b made of a second layer, and a transfer electrode 723 made of a third layer are formed on a semiconductor substrate 725 in the charge transfer regions 711 and the connection portions 712 with a gate insulating film 726 sandwiched between the transfer electrodes 721, 722a, 722b, 723 and the semiconductor substrate 725.
Between the transfer electrodes 721, 722a, 722b and 723 are formed interlayer insulating films 741 such as a thermal oxide film. All the transfer electrodes 721, 722a, 722b and 723 extend through the connection portions 712, and make electrical connection with a bus line (not illustrated) disposed around the semiconductor substrate 725.
When charges are transferred, a first phase driving pulse .phi.1 is applied to the transfer electrode 721 made of the first layer, a second phase driving pulse .phi.2 is applied to the transfer electrode 722a made of the second layer, a third phase driving pulse .phi.3 is applied to the transfer electrode 723 made of the third layer, and a fourth phase driving pulse .phi.4 is applied to the transfer electrode 722b made of the second layer. When a signal charge is read out from the photodiode region 710 to the charge transfer region 711, a read-out pulse .phi.TG in addition to a driving pulse is applied to one of those four transfer electrodes 721, 722a, 722b and 723, for instance, to the transfer electrode 723 made of the third layer.
As illustrated in FIGS. 2B and 2C, the four transfer electrodes 721, 722a, 722b and 723 and the semiconductor substrate 725 are covered with an interlayer insulating film 742 such as a thermal oxide film and an oxide film formed by CVD. In addition, a photoshield film 724 is formed on the interlayer insulating film 742 above the transfer electrodes 721, 722a, 722b and 723 for preventing lights from directly entering the charge transfer regions 711. As illustrated in FIG. 2C, the photoshield film 724 is formed with openings 728 between the transfer electrodes 721, 722a, 722b and 723 in the direction X1. The photoshield film 724 is grounded, or is designed to be fixed at a certain voltage, and has a function of only preventing lights from passing therethrough.
Thus, the secondly mentioned conventional progressive-scan image sensor has provided a three-layered electrode, four-phase driving type progressive-scan image sensor.
Hereinbelow is explained the thirdly mentioned conventional progressive-scan image sensor with reference to FIGS. 3A to 3C. FIG. 3A is a schematic view illustrating the thirdly mentioned conventional progressive-scan image sensor, FIG. 3B is a cross-sectional view taken along the line IIIB-IIIB, and FIG. 3C is a cross-sectional view taken along the line IIIC-IIIC in FIG. 3A.
The illustrated image sensor is of an interline type image sensor comprising photodiode regions 810, and charge transfer regions 811 for reading signal charges photoelectrically converted in the photodiode regions 810, and transferring the thus photoelectrically converted signal charges in a vertical transfer direction X1. The photodiode regions 810 are spaced away from one another in the direction X1. Hereinafter, spaces formed between the photodiode regions 810 in the direction X1 are referred to as connection portions 812.
Each of transfer electrodes is comprised of three sections per a pixel. Namely, as illustrated in FIGS. 3B and 3C, transfer electrodes 821h and 821i made of a first layer and a transfer electrode 822j made of a second layer are formed on a semiconductor substrate 825 in the charge transfer regions 811 and the connection portions 812 with a gate insulating film 826 sandwiched between the transfer electrodes 821h, 821i, 822j and the semiconductor substrate 825.
Between the transfer electrodes 821h, 821i and 822j is formed an interlayer insulating film 841 such as a thermal oxide film. The transfer electrode 822j made of the second layer is surrounded by the photodiode regions 810, and is pattered in the form of an island. The transfer electrodes 821h and 821i other than the island-shaped transfer electrode 822j extend through the connection portions 812, and make electrical connection with a bus line (not illustrated) disposed around the semiconductor substrate 825.
There is formed an electrically conductive wiring layer 830 over the transfer electrodes 821h, 821i and 822j in the direction X1. The electrically conductive wiring layer 830 makes electrical connection with the above-mentioned bus line, and further with the island-shaped transfer electrode 822j through a contact hole 827.
When charges are transferred, a first phase driving pulse .phi.1 is applied to the electrically conductive wiring layer 830 and hence the island-shaped transfer electrode 822j made of the second layer, a second phase driving pulse .phi.2 is applied to the transfer electrode 821i made of the first layer, and a third phase driving pulse .phi.3 is applied to the transfer electrode 821h made of the first layer. When a signal charge is read out from the photodiode region 810 to the charge transfer region 811, a read-out pulse .phi.TG in addition to a driving pulse is applied to one of the three transfer electrodes 821h, 821i and 822j, for instance, to the transfer electrode 822j through the electrically conductive wiring layer 830.
Since the electrically conductive wiring 830 is formed only for the purpose of applying driving pulses to the transfer electrodes, the electrically conductive wiring 830 does not have a function of completely preventing lights from entering the charge transfer regions 811. In an interline transfer type image sensor, a photoshield film 824 has to be formed covering the charge transfer regions 811 therewith in order to prevent lights from directly entering the charge transfer regions 811. Accordingly, there has to be formed the photoshield film 824 over the electrically conductive wiring layer 830 in the thirdly mentioned conventional image sensor, as illustrated in FIGS. 3B and 3C.
The thirdly mentioned conventional image sensor has to have totally four films, similarly to the firstly and secondly mentioned conventional image sensors: first and second layers for forming the transfer electrodes 821h, 821i and 822j thereof; a layer for forming the electrically conductive wiring layer 830 for applying driving pulses to the island-shaped transfer electrode 822j; and the photoshield film 824 for preventing lights from entering the photodiode regions 810.
As mentioned above, the conventional progressive-scan image sensors include totally four films. Specifically, the firstly and secondly mentioned image sensors include three films for forming transfer electrodes thereof and a film for forming a photoshield film thereof, and the thirdly mentioned image sensor includes two films for forming transfer electrodes thereof, a film for forming an electrically conductive wiring layer, and a film for forming a photoshield film thereof. The four-film structure causes problems as follows.
The first problem is as follows. The four-film structure causes an enormous interlayer capacity, which is accompanied of higher power consumption in charge transfer regions. Herein, power consumption is defined as CV.sup.2 f wherein C indicates a parasitic capacity of a transfer electrode, V indicates a driving voltage, and f indicates a driving frequency.
The second problem is as follows. The increased interlayer capacity causes driving pulses applied to transfer electrodes and a read-out pulse to become dull in a waveform thereof in accordance with a time constant defined as a product C.times.R where C indicates a parasitic capacity of a transfer electrode, and R indicates a resistance of a transfer electrode. This in turn causes a problem that improper transfer and/or incorrect read-out tends to occur.
The third problem is an increase in smear. As a cell becomes smaller in size, an aspect ratio becomes greater, and as a result, apparent steps also become larger. Hence, smear would be increased due to deterioration in sensitivity caused by improper light-collection and further due to reflection and diffusion of incident lights.
The fourth problem is as follows. As the steps become larger, it would become difficult to properly form transfer electrodes and a photoshield film. This causes a problem that transfer electrodes and a photoshield film tend to be short-circuited, resulting in a problem of reduction in a fabrication yield of a device.
The fifth problem is as follows. As the steps become larger, it would be difficult to properly form an on-chip color filter and/or an on-chip micro-lens formed subsequently formation of a photoshield film. This causes reduction in sensitivity, non-uniformity in sensitivity in a plane of a chip, and color mixture among pixels.
The sixth problem is that since there has to be formed at least totally four films, three films for forming transfer electrodes and a film for forming a photoshield film, the greater number of fabrication steps have to be carried out, resulting in an increase in fabrication costs.